The present invention relates to processes for the preparation of polymers, and more specifically to a polymerization process and to the polymer formed thereby. In one embodiment, the present invention relates to a stable free radical moderated process for producing a thermoplastic polymer resin or resins, that have narrow polydispersities, that is, narrow molecular weight distributions as defined by the ratio M.sub.w :M.sub.n, where M.sub.w is weight average molecular weight and M.sub.n is number average molecular weight, and easily controllable modality, from at least one monomer compound comprising heating for an effective period of time a mixture of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound under conditions such that all polymer chain formations are initiated at about the same time; cooling the mixture to effectively terminate the polymerization; isolating the thermoplastic resin product; and optionally washing and drying the polymer resins. The polymer resins produced by the process of the present invention in embodiments are essentially monomodal and in embodiments by repeating the heating step, that is, the combined initiation and polymerization step, provides a means for obtaining mixtures of monomodal polymer resins, that are compositionally the same resin type having characteristics of both narrow polydispersity and known or selectable modality. In another embodiment the process of the instant invention provides a means for conducting bulk or neat free radical polymerization processes on multikilogram or larger scales. The aforementioned embodiments may be accomplished in a one or single pot reactor environment. In another embodiment polymeric chain growth proceeds by a pseudoliving mechanism and can provide resins of variable molecular weights from very low to very high, for example, less than about 10,000 up to about 200,000 while maintaining narrow molecular weight distributions or polydispersities. In another embodiment block copolymers can be synthesized by the aforementioned stable free radical moderated free radical polymerization processes wherein each block formed is well defined in length by the reacted monomer and wherein each block formed possesses a narrow molecular weight distribution.
Of the known polymerization processes a preferred way to prepare polymers or copolymers having a narrow molecular weight distribution or polydispersity is by anionic processes. The use and availability of resins having narrow polydispersities in industrial applications is limited because anionic polymerization processes must be performed in the absence of atmospheric oxygen and moisture, require difficult to handle and hazardous initiator reagents and consequently such polymerization processes are generally limited to small batch reactors. In addition, the monomers and solvents that are used must be of high purity and anhydrous rendering the anionic process more expensive than alternatives which do not have these requirements. Thus, anionic polymerization processes are difficult and costly. It is desirable to have a free radical polymerization process that would provide narrow molecular weight distribution resins that overcome the shortcomings and disadvantages of the aforementioned anionic polymerization processes.
Free radical polymerization processes are chemically less sensitive to impurities in the monomers or solvents typically used and are completely insensitive to water. There has been a long felt need for an economical free radical polymerization process which is suitable for preparing narrow polydispersity resins by suspension, solution, bulk or neat, emulsion and related processes.
Copolymers prepared by free radical polymerization processes inherently have broad molecular weight distributions or polydispersities, generally greater than about five. One reason is that free radical initiators have half lives that are relatively long, from several minutes to many hours, and polymeric chains are not all initiated at the same time and which initiators provide growing chains of various lengths at any time during the polymerization process. Another reason is that the propagating chains in a free radical process can react with each other in processes known as coupling and disproportionation, both of which are chain terminating reactions. In doing so, chains of varying lengths are terminated at different times during the reaction process which results in resins comprised of polymeric chains which vary widely in length from very small to very large. If a free radical polymerization process is to be enabled for producing narrow molecular weight distributions, then all polymer chains must be initiated at about the same time and premature termination by coupling or disproportionation processes must be avoided.
Otsu et.al., in Makromol Chem., Rapid Commun., 3, 127 (1982), introduced the use of iniferters as a means of producing block copolymers by a free radical polymerization process. A mechanism proposed for the reaction suggested that a pseudoliving propagating free radical chain exists and that it continues to grow with time. There are two major drawbacks associated with using iniferters. Iniferters tend to react very slowly and the percent conversion or degree of polymerization of monomer to polymer is low, for example, about 40 percent even after 20 hours of reaction time. Another drawback is that the free radical trap that caps the end of the growing chain has the ability to initiate new chains at any time during the course of the reaction, see for example, S. R. Turner, R. W. Blevins, in Polymer Reprints, 29(2), September 1988. This initiation leads to new chains being initiated at various times during the polymerization and consequently leads to broadening of the polydispersity. Although the approach in the aforementioned reference of Otsu was novel in using pseudoliving free radical propagating chains, it was not applicable to the synthesis of narrow molecular weight distribution resins particularly for polymers with high molecular weights.
The use of stable free radicals are well known as inhibitors of free radical polymerizations, see for example, G. Moad et.al., Polymer Bulletin 6, 589 (1982). Studies by, for example, G. Moad et.al. J. Macromol. Sci.-Chem., A17(1), 51(1982) have reported on the use of stable free radicals as inhibitors of free radical polymerizations performed at low temperatures, for example, below 90.degree. C. and at low monomer to polymer conversions. Little is known concerning the reaction of stable free radical agents at higher temperatures and at high monomer to polymer conversions.
In a hypothetical free radical polymerization of styrene, in which chains are continually initiated over the course of the polymerization, and where chain termination by coupling processes is also occurring, calculations as described in, for example, G. G. Odian, Principles of Polymerization, pages 280-281, 2nd Ed., John Wiley & Sons, 1981 have shown that the narrowest polydispersity that one can theoretically possibly obtain is 1.5. In practice, polydispersities much greater than 1.5 are actually obtained. Polydispersities of between 2.0 and 2.4 are typical for free radical homopolymerizations of styrene. In the case of copolymer systems, polydispersities of greater than 4 are generally obtained.
The stable free radical polymerization system of the instant invention may readily afford polydispersities of between 1.15 and 1.25 for polystyrene and as low as 1.5 for various copolymer systems. Stable free radical polymerization systems of the instant invention afford polydispersities that are comparable to those obtained in anionic polymerizations.
In a patentability search report the following patents were recited:
U.S. Pat. No. 4,581,429 to Solomon et al., issued Apr. 8, 1986, discloses a free radical polymerization process which controls the growth of polymer chains to produce short chain or oligomeric homopolymers and copolymers including block and graft copolymers. The process employs an initiator having the formula (in part)=N--O--X, where X is a free radical species capable of polymerizing unsaturated monomers. The molecular weights of the polymer products obtained are generally from about 2,500 to 7,000 having polydispersities generally of about 1.4 to 1.8, at low monomer to polymer conversion. The reactions typically have low conversion rates and use relatively low reaction temperatures of less than about 100 degrees C., and use multiple stages.
U.S. Pat. No. 5,059,657 to Druliner et al., issued Oct. 22, 1991, discloses a polymerization process for acrylic and maleimide monomers by contacting the monomers with a diazotate, cyanate or hyponitrite, and N-chlorosuccinimide, N-bromosuccinimide or a diazonium salt. The polymer produced can initiate further polymerization, including use in block copolymer formation.
In free radical polymerization reaction processes of the prior art, various significant problems exist, for example difficulties in predicting or controlling the polydispersity and modality of the polymers produced. These free radical polymerization processes invariably produce polymers with high weight average molecular weights (M.sub.w) and low number average molecular weights (M.sub.n) resulting in broad polydispersities. Further, bulk or neat free radical polymerization processes of the prior art are prone to generating excessive quantities of heat since the polymerization reaction is exothermic and as the viscosity of the reaction medium increases dissipation of heat becomes more difficult. This is referred to as the Trommsdorff effect as discussed and illustrated in Principles of Polymerization, G. Odian, 2nd Ed., Wiley-Interscience, N.Y., 1981, page 272, the disclosure of which is entirely incorporated herein by reference. Moreover, the exothermic nature of free radical polymerization processes is often a limitation that severely restricts the concentration of reactants or the reactor size upon scale up.
It is known to form resins by bead suspension polymerization reference for example U.S. Pat. Nos. 4,601,968 and 4,609,607, the disclosures of which are totally incorporated herein by reference.
Illustrated in now U.S. Pat. No. 5,274,057, the disclosure of which is incorporated by reference herein in its entirety, is that free radical suspension polymerization reactions may also lead to undesirable deposits of polymer on the agitator, baffles, heating coils and reactor walls. In some situations, the suspension coalesces during the reaction producing large deposits of undesirable polymeric gel material which is difficult, expensive and hazardous to remove from the reactor.
Further, gel body formation in conventional free radical polymerization processes may result in a broad molecular weight distributions and/or difficulties encountered during filtering, drying and manipulating the product resin.
These and other disadvantages are avoided, or minimized with the processes of the present invention.
Thus, there remains a need for processes for the preparation of narrow polydispersity polymeric resins by economical and scalable free radical polymerization techniques and which polymers retain many or all of their desirable physical properties, for example, hardness, low gel content, processability, clarity, high gloss durability, and the like, while avoiding the problems of gel formation, exotherms, volume limited and multi-stage reaction systems, purification, performance properties of the polymer resin products, and the like associated with prior art free radical polymerization methodologies.
The polymerization processes and thermoplastic resin products of the instant invention are useful in many applications, for example, as a variety of specialty applications including toner resins used for electrophotographic imaging processes or where monomodal or mixtures of monomodal narrow molecular weight resins or block copolymers with narrow molecular weight distribution within each block component are suitable such as in thermoplastic films and coating technologies.